Though the cost of nitrogen and other fertilizer has dropped slightly from the record-high mark in 2008, paying particularly close attention to soil fertility levels and crop nutrient requirements is more important than ever. Growers need to come as close as possible to figuring out the formula for “just enough” fertilizer application to achieve optimum yield.

Recommendations for N, P and K in sunflower are based on the amount of nutrients present in the first two feet of soil. The recommendation for N stands at 50 lbs in the top two feet of soil for every 1,000 lbs of desired yield. But because these recommendations were based largely on studies done in the late 1970s and early 1980s, many soil scientists stress the importance of going well beyond university fertility recommendation tables to determine the best plan of action concerning their crops’ needs.

The nutrient content of the soil, as determined by a soil test, is the only practical way to predict probability of a response to applied nutrients. Information garnered from soil testing is also the most reliable means for growers to save money and obtain projected yield goals. With soil levels determined, growers save by avoiding unnecessary application. Years ago, when fertilizer was inexpensive, growers were of the mindset that adding a little fertilizer couldn’t hurt if they get some yield gain. But that has changed dramatically in the past few years, particularly because of nitrogen costs.

The technology associated with soil sampling has improved greatly over the past few years. Variable-rate application options help growers maximize fertilizer rates and costs. Grid sampling and zone management are ways crop consultants and soil scientists can customize fertilizer recommendations for specific fields.

Dennis Berglund with Centrol Ag Consulting, Twin Valley, Minn., says his company began utilizing grid soil sampling in Minnesota/eastern North Dakota Red River Valley sugarbeet acres in the mid-1990s. Grid sampling is a way of collecting data in sections to allow for variable rate application. In other words, tests from different areas would determine where fertilizer is needed, and saving money by putting less fertilizer in areas with sufficient nutrient base. Like sunflower, sugarbeets are a nitrogen-sensitive crop.

Grid sampling a section of land, for example, creates a square grid comprising up to 36 samples. “This was very popular at one point and is still used,” Berglund explains. “But we end up with a lot of soil samples, making it costly and time consuming. Now what we do more often is zone sampling.”

Zone management utilizes a combination of information sources such as satellite images, topography and yield map data from previous crop years to create soil maps. This method improves the soil sampling data in many ways — including taking less samples (four or five), therefore being less costly and taking less time. Satellite imagery also follows the contour of the land instead of a flat square as in grid sampling.

Brett Peterson, crop consultant with Centrol based in central North Dakota, says zone management has drastically changed the world of soil sampling. Satellite imagery breaks the field into three separate zones — higher yielding acres, average and poor — based on field data of vegetation from the past 10 years. Soil samples give information on the breakdown of the nutrient profile in each of the zones. “Our goal is to increase that green area because that’s where the best potential is for high yield, meaning that’s the best soil,” explains Peterson. “It’s taking the best parts of a field and making them better.”

Flat fertilization rates don’t make sense from an environmental or economic standpoint. Why apply fertilizer to areas that don’t need it or have poor potential for growth? In geographical areas with high variability of both the contour of the land and soil types, variable-rate application is a huge advantage in maximizing yield.

Brian Michels, crop advisor and owner of Production Service, Inc., near Mohall, N.D., has worked with zone management for a few years. He sees the benefit of this practice for producers as a soil management system, with emphasis on “system.” There are a number of tools available to effectively determine the nutrient needs of soil.

During this past growing season, Michels enlisted the services of MZB Technologies located in Watertown, S.D. MZB utilizes a Veris cart in the system of zone management. A Veris cart, developed by a company in Salina, Kan., is a small two-wheeled cart pulled across a field, collecting electrical conductivity (EC) data from the soil and geo-references them using a GPS receiver. EC is a measure of the ability of a material to transmit (conduct) an electrical charge. The data will determine how various soil types in a field differ in their ability to conduct electricity. Michels explains that, for instance, heavier soil conducts more electricity than sandy soil. Likewise, salty soil conducts more electricity than non-salty soil.

As the cart is pulled through the field, one pair of coulter-electrodes injects electrical current into the soil, while the other coulter-electrodes measure the voltage drop. While these coulter-electrodes only need to penetrate the soil a few inches, the electrical arrays employed by the Veris system investigate the soil to a depth of approximately 36”. The EC data collected by the Veris cart produce a map of soil texture variability. Wade Marzhan, of MZB Technologies, says his company has three units that they use in their zone management programs. “As a small current is injected into the soil, the unit is trying to complete a circuit,” Marzhan explains. “It’s reading resistance of the flow of electricity and converts it to conductivity.”

Essentially, the Veris cart is used to determine where to take soil samples to get a more accurate reading of what types of soil are present. The process is a fast and effective measurement of soil properties comparative to about 50 samples per acre taken in a conventional manner. Conventional soil sampling and lab analysis at this rate would be cost-prohibitive.

Michels and Marzhan stress that the soil mapping generated by the Veris cart is not a stand-alone component. Electrical conductivity of the soil, along with GPS-guided topography mapping and yield potential, are used to create accurate management zones. Fertilizer can be variably applied according to best management practices in each of the different productivity zones, maximizing each acre.

All of the information is gathered in order to generate prescription maps for the growers’ fertilizer plans. “We’re helping the grower find the most productive areas where the fertilizer needs to go,” Michels notes. “We’ve dealt with low-protein wheat in this area in recent years, and in many cases it’s because the wheat runs out of nitrogen. With zone management, the producer can be more efficient at placing the right amount of fertilizer in the areas of the specific field that are capable of producing.” Based on the zone management and variable rate application of fertilizer, Michels has helped growers produce a crop of wheat that showed more protein response, saving money and pushing yields.

This spring, Michels will be testing the zone management system on sunflower ground to get a feel for how it works. But he figures with an N-sensitive crop like sunflower, the technology will have big payoffs.

Zone management technology has also come in to play for variable seeding rates. Satellite images can show, for example, the sandy hilltop areas that are prone to sparse production — and conversely, the lower ground with dense, fertile soil where production has excellent potential.

Achieving consistent plant stands continues to be a challenge for sunflower growers and a yield-limiting factor. The maps generated with zone management can assist the grower in adjusting seeding rates and, in turn, save input cost and increase yield. While most of the testing to date has been done with corn, sunflower could benefit greatly from more research into variable seeding rates using zone management. - Sonia Mullally